This invention relates to a process for coating in one step a bake hardenable, bare or metallic coated, steel sheet with an induction cured thermosetting coating. More particularly, the coating is formed from an aqueous solution containing a thermosetting polymer and a chromium compound. The coating is formable, impervious to moisture, resistant to dissolution of the chromium and is capable of being bonded with a top coating of paint or adhesive.
It is known to pretreat and/or prepaint steel substrates for improving corrosion resistance prior to post painting. The pretreatment generally is a two step process including initially applying a dried-in-place inorganic chromium primer having a weight of about 0.75 g/m.sup.2 or more followed by baking of a solvent based organic liquid coating, e.g., epoxy containing a silicate, having a weight of about 0.5-1.1 g/m.sup.2. The chromium layer beneath the organic layer is desirable because it retards the formation of corrosion products by inhibiting the hydration of metallic oxides. The organic topcoat provides additional barrier protection and is necessary to prevent dissolution of the chromium from the primer layer.
Electroplating steel sheets with zinc nickel forms a relatively brittle metal coating that may have inferior adherence during stamping of the sheet and subsequent poor corrosion resistance. Applying an organic coating over the zinc nickel coating prior to painting of the sheet acts a barrier film inhibiting catastrophic corrosion during field service. Without the organic coating, it would be necessary to increase the thickness of the metallic coating to insure the necessary corrosion resistance. An increase of the metallic coating thickness, however, could adversely affect welding and formability of the sheet. U.S. Pat. No. 4,971,636 discloses coating a zinc coated steel sheet with a composite coating of chromium and epoxy resin using a two step process. The zinc coated steel is rinsed with a chromic acid solution, dried, coated with an aqueous solution containing epoxy and silica and the epoxy coated steel then is baked.
In a more recent development, U.S. Pat. No. 5,001,173; incorporated herein by reference, discloses a steel sheet pretreated in a one step process with an organic coating containing chromium. The coating is applied from a solution of 2-25 wt. % of an organic resin component containing at least one water dispersible or emulsifiable epoxy resin or a mixture of resins containing more than 50 wt. % of at least one water dispersible or emulsifiable epoxy resin, 0.5-5 wt. % chromium trioxide and 25-97 wt. % water. The liquid coated sheet is dried at a baking temperature of 93.degree.-260.degree. C. forming a flexible crack resistant coating impervious to moisture. The epoxy coating can be welded and adhesively bonded with an outer top coating of paint.
In recent years, automobile manufacturers have required steel sheet to have a property known as bake hardenability. After the steel is pretreated with paint primer, lubricant, and the like, the steel is formed into a part. Thereafter, the formed part is painted and baked at an elevated temperature to cure the paint. Certain steels have a capacity for a significant increase in yield strength by the combination of work hardening during forming the part and strain aging when the work hardened steel is subjected to an overaging treatment, i.e., heated to an elevated temperature. By bake hardenable is meant a steel that exhibits this increase of yield strength when being formed into a part and subsequently heated at an elevated temperature to cure the paint. Bake hardenable steels have become popular with automobile manufacturers because the increase in yield strength increases dent resistance of the part without sacrificing formability and permits use of relatively thin steel sheets thereby assisting in weight reduction of a vehicle. Unfortunately, formability and bake hardenability of a steel may deteriorate if the steel is heated to a temperature of about 165.degree. C. prior to forming the part, especially if the steel is pretreated with a two step process. Steels heretofore pretreated with an organic coating prior to forming the steel into a part require the pretreated steel to be baked at an elevated temperature. For a pretreatment that includes hexavalent chromium in the liquid coating, the baking temperature may be insufficient to completely reduce the unstable chromium in the liquid to stable trivalent chromium in the cured coating. When not sufficiently reduced, chromium tends to dissolve during subsequent processing steps causing contamination of cleaning, pretreatment and rinsing solutions. This contamination is undesirable because the waste solutions are environmentally unacceptable and disposition is expensive. Chromium contamination in these solutions also may inhibit cleaning and pretreatment of the steel leading to poor paint adhesion and/or corrosion resistance on the vehicle. For pretreatments requiring a two step process, the steel tends to lose its formability and bake hardenability properties at temperatures above 165.degree. C. because the steel must be heated twice. That is, the steel is heated to dry-in-place the inorganic chromium layer and then is baked to cure the organic coating. For a two step pretreating process, the baking temperature must be restricted to less than 165.degree. C.
Accordingly, there remains a need for a steel sheet pretreated with a chromium bearing organic coating wherein the coating provides good corrosion resistance and the steel base metal retains good bake hardenable properties. There remains a further need for a pretreated bake hardenable steel sheet wherein chromium in the organic coating will not dissolve during subsequent processing of the sheet. Ideally, the organic coating needs good formability, enhanced painting characteristics and sufficient lubricity so that additional lubricant is not required during stamping of the coated steel into a part.